LAS VEGAS -- Protect yourself at all times. It is the oldest adage in boxing. Victor Ortiz did not do that, and he paid the ultimate price.

Floyd Mayweather Jr., who had dominated the first three rounds, looked brilliant in his first fight in 16 months, but that brilliance may be overshadowed by the way he scored a stunning knockout with one second left in the fourth round Saturday night to win a welterweight title at the MGM Grand Garden Arena.

Some at ringside called it a sucker punch from Mayweather. Others blamed referee Joe Cortez, who did not have his eye on the action. Whatever you want to call it, Mayweather drilled Ortiz in a triumphant return to the ring.

Victor Ortiz let his guard down for just a moment in the fourth round, which was long enough for Floyd Mayweather Jr. to drop Ortiz for the knockout. Ethan Miller/Getty Images

Ortiz was getting tagged repeatedly by Mayweather right hands for most of the fight. Mayweather was making it look oh-so easy, as he usually has during a career that obviously will send him to the Hall of Fame.

But in the fourth round, Ortiz found some success. He landed a couple of hard shots that had Mayweather shaking his head, as if say he did not feel them. But then Ortiz drove Mayweather into the corner post.
Instead of following up with punches, Ortiz drove his head toward Mayweather's face as if to intentionally head-butt him. Cortez immediately called time and deducted a point from Ortiz.

Then the fight turned somewhat surreal.

As Cortez was calling time, Ortiz -- seemingly acknowledging his wrongdoing -- hugged Mayweather in the corner and even appeared to kiss him.

Then Cortez waived them back together and they touched gloves. But with Cortez looking away from the fighters, Mayweather unloaded a left hook and a flush right hand to Ortiz's face.

Ortiz dropped and was unable to beat Cortez's count as the crowd of 14,687 -- clearly an Ortiz house -- went absolutely wild.

"In the ring, you have to protect yourself at all times," Mayweather said. "After it happened, we touched gloves and we were back to fighting and then I threw the left hook and right hand after the break. You just gotta protect yourself at all times."

During the lead-up to the fight, Ortiz's trainer, Danny Garcia, had accused Mayweather of using his elbows and being a dirty fighter.

But it was Ortiz (29-3-2, 22 KOs) who turned dirty with the head-butt, not Mayweather, who was well within his rights to throw the punches that ended the fight.

Mayweather (42-0, 26 KOs) did not have a lot of time to celebrate and was soon questioned about what had happened by HBO's Larry Merchant during an in-ring interview.

"He did something dirty," Mayweather said. "His corner said I was dirty, and he did something dirty. All along, his corner was saying I was dirty, but I won the fight."

At that point Mayweather's interview with Merchant degenerated. Mayweather began to get into Merchant's face, saying, "I'm going to do you a favor and let you talk to Victor Ortiz, because you've never given me a fair shake. You ain't s---."

"If I was 50 years younger, I would kick your ass," Merchant, 80, said back to him before turning to Ortiz.

HBO will replay the fight next Saturday night at 9:45 ET/PT.

Mayweather, who made a minimum $25 million, walked away from Merchant, clearly in no mood to dissect what had happened or to address his overwhelming performance, one which he deserves credit for as he took apart a man 10 years younger than him (Ortiz is 24) while coming off the long, self-imposed layoff.

He also faced a significant weight disadvantage.

Ortiz, who was making the first defense of the belt he won via decision in a fight of the year candidate with Andre Berto in April, stepped onto the unofficial HBO fight-night scale at 164 pounds. Mayweather was 150 pounds.

It made no difference, as Mayweather claimed his seventh world title covering five weight classes.

Ortiz was, of course, not happy with what had happened, even if he has only himself to blame for the head-butt that led to the knockout.

"I took a break by the referee and I obeyed exactly what I was told," said Ortiz, who made $2 million. "And then, boom, he blindsided me. I'm not a dirty fighter and I apologize for the head-butt. I came to entertain the fans and I think they were entertained. There was a miscommunication with the referee, and neither he nor I are perfect."

This is true. But, in the ring anyway, Mayweather sure is.

Dan Rafael is the boxing writer for ESPN.com. Follow him on Twitter @danrafaelespn.